A1N inter-layers in III-N material grown on DBR/silicon substrate

ABSTRACT

A DBR/gallium nitride/aluminum nitride base grown on a silicon substrate includes a Distributed Bragg Reflector (DBR) positioned on the silicon substrate. The DBR is substantially crystal lattice matched to the surface of the silicon substrate. A first layer of III-N material is positioned on the surface of the DBR, an inter-layer of aluminum nitride (AlN) is positioned on the surface of the first layer of III-N material and an additional layer of III-N material is positioned on the surface of the inter-layer of aluminum nitride. The inter-layer of aluminum nitride and the additional layer of III-N material are repeated n-times to reduce or engineer strain in a final III-N layer.

FIELD OF THE INVENTION

This invention relates in general to the growth of III-N material on asilicon substrate and more specifically to the formation of a DBR on thesilicon substrate and III-N material with one or more aluminum nitrideinter-layers grown thereon.

BACKGROUND OF THE INVENTION

In the semiconductor industry, it is known that growing a III-Nmaterial, such as GaN, on a silicon substrate is difficult due in largepart to the large crystal lattice mismatch (−16.9%) and the thermalmismatch (53%) between silicon and GaN. It is also known that LEDdevices built on silicon substrates suffer from absorption of emittedlight by the silicon substrate. Also, during much of the growth processthere must ideally be no exposed silicon surface due to detrimentalreaction between the silicon and the various MBE process gasses, i.e. N₂plasma, NH₃ and metallic Ga. Also in the case where other growthprocesses are used, such as MOCVD process gasses (NH₃, H₂, TMGa, etc.).Reaction of silicon with process gasses usually results in etching ofsilicon (H₂), formation of nitrides (NH₃), or severe reaction andblistering (Ga precursors).

In the prior art, one method of solving the light absorption problem isto fabricate the LED on a silicon substrate and then bond the finishedLED on a reflective coating and remove the silicon substrate. Generally,the top layer of the resulting structure is roughened to improve lightextraction efficiency. However, this is a long and work intensiveprocess.

It would be highly advantageous, therefore, to remedy the foregoing andother deficiencies inherent in the prior art.

Accordingly, it is an object of the present invention to provide a newand improved DBR/gallium nitride/aluminum nitride base on a siliconsubstrate.

It is another object of the present invention to provide a new andimproved DBR/gallium nitride/aluminum nitride base on a siliconsubstrate that greatly reduces strain in III-N material grown on thebase.

It is another object of the present invention to provide a new andimproved DBR/gallium nitride/aluminum nitride base that reduces oreliminates absorption of light emitted by an LED formed thereon.

It is another object of the present invention to provide a new andimproved DBR/gallium nitride/aluminum nitride base that reduces oreliminates the problem of possible damage to the silicon substrate withprocess gasses.

SUMMARY OF THE INVENTION

Briefly, the desired objects and aspects of the instant invention arerealized in accordance with a DBR/gallium nitride/aluminum nitride basegrown on a silicon substrate. The DBR is substantially crystal latticematched to the surface of the silicon substrate. A first layer of III-Nmaterial is positioned on the surface of the DBR, an inter-layer ofaluminum nitride (AlN) is positioned on the surface of the first layerof III-N material and an additional layer of III-N material ispositioned on the surface of the inter-layer of aluminum nitride. Theinter-layer of aluminum nitride and the additional layer of III-Nmaterial are repeated n-times to reduce or engineer strain in a finalIII-N layer.

The desired objects and aspects of the instant invention are furtherachieved in accordance with a preferred method of growing a DBR/galliumnitride/aluminum nitride base on a silicon substrate. The methodincludes growing a Distributed Bragg Reflector (DBR) on the siliconsubstrate. The DBR is substantially crystal lattice matched to thesurface of the silicon substrate. A first layer of III-N material ispositioned on the surface of the DBR, an inter-layer of aluminum nitride(AlN) is positioned on the surface of the first layer of III-N materialand an additional layer of III-N material is positioned on the surfaceof the inter-layer of aluminum nitride. The inter-layer of aluminumnitride and the additional layer of III-N material are repeated n-timesto reduce or engineer strain in a final III-N layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further and more specific objects and advantages ofthe instant invention will become readily apparent to those skilled inthe art from the following detailed description of a preferredembodiment thereof taken in conjunction with the drawings, in which:

FIG. 1 is a simplified layer diagram of a DBR on a silicon substrate, inaccordance with the present invention;

FIG. 2 is a chart illustrating different materials and the indexes ofrefraction;

FIG. 3 is a simplified layer diagram illustrating a method of growingDBR/gallium nitride/aluminum nitride on a silicon substrate, inaccordance with the present invention; and

FIG. 4 is a simplified layer diagram of the DBR/gallium nitride/aluminumnitride on the silicon substrate of FIG. 3 with a photonic structureformed thereon.

DETAILED DESCRIPTION OF THE DRAWINGS

It is known in the semiconductor industry that the fabrication of LEDson silicon substrates is the most efficient because of the expense andwide use and established technology in the use of silicon. However, asstated above, it is also known that LED devices built on siliconsubstrates suffer from absorption of emitted light by the siliconsubstrate. LEDs emit light in all directions and any light directed atthe silicon substrate is substantially lost since it is absorbed by thesilicon substrate. Prior art has placed reflective surfaces on one sideof the LED and removed the substrate so that substantially all light isemitted in one direction. This however is a very tedious and workintensive process.

Turning to FIG. 1, a simplified layer diagram is illustrated of anexample of a Distributed Bragg Reflector (DBR) 12 grown on a siliconsubstrate 10, in accordance with the present invention. It will beunderstood that substrate 10 is or may be a standard well known singlecrystal wafer or portion thereof generally known and used in thesemiconductor industry. Single crystal substrates, it will beunderstood, are not limited to any specific crystal orientation butcould include (111) silicon, (110) silicon, (100) silicon or any otherorientation or variation known and used in the art. The Si (100) and(111) substrates could also include various miscuts with nominal valuebetween 0 and 10° in any direction. In the present invention a (111)silicon single crystal substrate is preferred because of the simplicityof further epitaxial growth.

As is known in the art, DBRs consist of a plurality of pairs of layersof material, with each pair forming a partial mirror that reflects someof the light incident upon it. In FIG. 1, DBR 12 is illustrated ashaving three pairs 14 with each pair including layers 15 and 16.Reflection or the mirror effect is produced by choosing the materials oflayers 15 and 16 with a substantial difference in the refractiveindices. Also, the number of pairs 14 are chosen to provide the mostoverall reflection for the most efficient, inexpensive or practicaldevice.

Referring additionally to the chart of FIG. 2, the index of refractionfor several materials are included with the index of refraction at 450nm (the general wavelength of light for LEDs formed from III-Nmaterials). Also illustrated in the chart of FIG. 2 is the difference inrefractive indices for some pairs of the materials. From the materialsincluded, it was determined that pairs of rare earth oxide and silicon(REO/Si) layers provide the largest refractive index difference (2.05)and therefore provide the best DBR pair.

Generally layers 15 and 16 are grown epitaxially on silicon substrate 10and on each other as layers of single crystal material. Various rareearth oxides have a crystal lattice spacing that can be substantiallymatched to silicon with very little strain. For example, Gd₂O₃ has acrystal lattice spacing (a) of 10.81 Å, Er₂O₃ has a crystal latticespacing (a) of 10.55 Å, Nd₂O₃ has a crystal lattice spacing (a) of 11.08Å, and silicon has a double spacing (2a) of 10.86 Å. Thus, REOa-Si2aherein defined as a “substantial crystallographic match”. Further, thecrystal lattice spacing of the REO layers can be varied by varying thecomposition of the constituents so that a closer match to GaN can beachieved at the opposite or upper surface.

Because the REO layers and the Si layers are substantially latticematched, the first and last layers of DBR 12 can be either a REO layeror a Si layer. Also, it should be noted that because the Si layers inDBR 12 are very thin very little impinging light will be absorbed. Inthe example illustrated, pairs 14 of layers 15 and 16 are repeated threetimes, which forms a DBR mirror that is highly effective (90% ofincident light is reflected) due to the larger refractive indexdifference between REO and silicon. It will be understood that more orfewer pairs 14 can be incorporated if a greater or lesser effectivereflection is desired, e.g. 2-5 pairs. Also, each pair can include thesame or a different REO material.

Turning to FIG. 3, a simplified layer diagram is illustratedrepresenting several steps in a process of growing III-N material and inthis preferred example GaN on a silicon substrate 10, in accordance withthe present invention. A DBR 12 including rare earth oxide (REO) isepitaxially grown on silicon substrate 10. The crystal lattice spacingof DBR 12 can be varied by varying the composition of the constituents,which allows for strain engineering of the silicon wafers. Generally,the REO material closest to or adjacent silicon substrate 10 will have acrystal spacing closest to the crystal spacing of silicon while REOmaterials adjacent the opposite side of DBR 12 will have a crystalspacing closer to the crystal spacing of materials grown on the surface.Strain engineering mitigates the stresses formed during growth of III-Nmaterials on these substrates.

It should be noted that rare earth oxide is impervious to MBE processgasses, i.e. N₂ plasma, NH₃ and metallic Ga, which is the preferredgrowth process in this invention. Also, in the event that other growthprocesses are used, such as the MOCVD process, the rare earth oxide isalso impervious to MOCVD process gasses (NH₃, H₂, TMGa, etc.). Reactionof silicon with process gasses usually results in etching of silicon(H₂), formation of nitrides (NH₃), or severe reaction and blistering (Gaprecursors). Thus silicon substrate 10 is protected from damage causedby generally all growth process gasses by rare earth oxide layers in DBR12.

A first gallium nitride (GaN) layer 20 is epitaxially grown on DBR 12preferably by an MBE process. Generally, first GaN layer 20 will be in arange of 50 nm to 100 nm thick, although thicker or thinner layers canbe grown. Because there will still be some strain in GaN layer 20, i.e.the crystal lattice junction with DBR 12 produces some strain, a thinnerlayer 20 of GaN is preferred.

A very thin inter-layer of aluminum nitride (AlN) 22 is epitaxiallygrown on first GaN layer 20 to further reduce the strain. Preferably,AlN inter-layer 22 is in a range of approximately 1 nm to approximately10 nm thick but for certain applications thicker or thinner films can begrown. Also, AlN inter-layer 22 can be grown using either a low or ahigh temperature process. A second layer 24 of GaN is epitaxially grownon AlN inter-layer 22. A second inter-layer of AlN (not shown) is grownon second GaN layer 24 and this process is repeated n times or until thestrain in the upper or final GaN layer has been reduced to an acceptablelevel. Basically, the strain formed during the growth of the GaN iscontrolled by insertion of the thin inter-layers of AlN, each of whichallows the following layer of GaN to be under compressive stress due tothe pseudomorphic growth at the interface. Repeating the process (i.e.the alternating growth of layers 20, 22 and 24) n times can be used tofurther reduce or engineer strain in the final GaN or III-N layer. Also,it should be noted that since each additional layer of GaN grown on thenext inter-layer of AlN has less strain, each additional layer can begrown thicker if desired.

Turning to FIG. 4, DBR 12, GaN layer 20/AlN inter-layer 22/GaN layer 24(repeated n times) is illustrated with a III-N LED or HEMT structure 30formed thereon. Structure 30 is illustrated as a single layer forconvenience but it should be understood that III-N LED or HEMT structure20 includes the growth of one or more typical layers. For example, whenstructure 30 is an LED the layers may include for example, i-GaN, n-GaN,active layers such as InGaN/GaN, electron blocking layers, p-GaN, andother inter-layers used in the formation and performance of LED(especially photonic LED) devices. When structure 30 is an HEMT thelayers may include the growth of one or more typical layers, includingfor example, i-GaN, AlN, AlGaN, GaN, and other inter-layers used in theformation and performance of HEMT devices.

Thus, new and improved methods for the growth of III-N material andphotonic devices on a silicon substrate are disclosed. The new andimproved methods for the III-N material include the growth of asubstantially crystal lattice matching DBR on the silicon substrate andn repetitions of the growth of thin AlN inter-layers in the III-Nmaterial to further reduce or engineer the strain. The DBR reflectslight generated in LED or HEMT structures away from the siliconsubstrate and back in the direction chosen for emission. Also, the DBReliminates or greatly reduces the problem of possibly damaging thesilicon substrate with process gasses. New and improved LED and/or HEMTstructures can be substantially lattice matched and thermally matched bythe new process on a silicon substrate.

Various changes and modifications to the embodiments herein chosen forpurposes of illustration will readily occur to those skilled in the art.To the extent that such modifications and variations do not depart fromthe spirit of the invention, they are intended to be included within thescope thereof which is assessed only by a fair interpretation of thefollowing claims.

Having fully described the invention in such clear and concise terms asto enable those skilled in the art to understand and practice the same,the invention claimed is:
 1. A DBR/gallium nitride/aluminum nitride basegrown on a silicon substrate comprising: a single crystal siliconsubstrate; a Distributed Bragg Reflector (DBR) positioned on the siliconsubstrate, the DBR being substantially crystal lattice matched to thesurface of the silicon substrate, the DBR including multiple layers ofsingle crystal rare earth oxide and alternate layers of single crystalsilicon; a first layer of single crystal GaN positioned on the surfaceof the DBR, the first layer of GaN including a layer in a range of 50 nmto 100 nm thick; and an inter-layer of single crystal aluminum nitride(AlN) positioned on the surface of the first layer of single crystalGaN, the inter-layer of aluminum nitride having a thickness in a rangeof 1 nm to 10 nm thick, and an additional layer of single crystal GaNpositioned on the surface of the inter-layer of single crystal aluminumnitride, the inter-layer of single crystal aluminum nitride and theadditional layer of single crystal GaN repeated n-times to reduce orengineer strain in a final III-N layer; and one of an LED structure andan HEMT structure formed in the final III-N layer, and the DistributedBragg Reflector (DBR) being positioned to direct emitted light from thesubstrate.
 2. The DBR/gallium nitride/aluminum nitride base grown on thesilicon substrate as claimed in claim 1 wherein the HEMT structureincludes at least one layer of i-GaN, AlN, AlGaN, or GaN.
 3. TheDBR/gallium nitride/aluminum nitride base grown on the silicon substrateas claimed in claim 1 wherein at least one of the multiple layers ofrare earth oxide included in the DBR include a composition of multiplerare earth oxides one of graded to bridge the multiple rare earth oxidesor stepped to have an abrupt change in the rare earth oxides.
 4. TheDBR/gallium nitride/aluminum nitride base grown on the silicon substrateas claimed in claim 3 wherein the composition including multiple rareearth oxides includes a first rare earth oxide adjacent the siliconsubstrate having a crystal lattice spacing substantially matching adouble lattice spacing of silicon and a second rare earth oxide adjacentthe first layer of single crystal GaN having a crystal lattice spacingsubstantially matching a crystal lattice spacing of the first layer ofsingle crystal GaN.
 5. The DBR/gallium nitride/aluminum nitride basegrown on the silicon substrate as claimed in claim 1 wherein the one ofan LED structure and an HEMT structure includes an LED, and theDistributed Bragg Reflector (DBR) is grown and positioned to reflectlight emitted by the LED from the substrate toward the LED.
 6. TheDBR/gallium nitride/aluminum nitride base grown on the silicon substrateas claimed in claim 1 wherein the LED structure includes at least onelayer of i-GaN, n-GaN, an active layer, an electron blocking layer, orp-GaN.
 7. A method of growing a DBR/gallium nitride/aluminum nitridebase on a silicon substrate comprising the steps of: providing a singlecrystal silicon substrate; epitaxially depositing a Distributed BraggReflector (DBR) on the silicon substrate, the DBR including multiplelayers of single crystal rare earth oxide and alternate layers of singlecrystal silicon, and the DBR being substantially crystal lattice matchedto the surface of the silicon substrate; epitaxially depositing a firstlayer of single crystal GaN on the surface of the DBR, includingepitaxially depositing the first layer of GaN in a range of 50 nm to 100nm thick; epitaxially depositing an inter-layer of single crystalaluminum nitride (AlN) on the first layer of single crystal GaN,including epitaxially depositing the inter-layer of single crystalaluminum nitride in a range of 1 nm to 10 nm thick; epitaxiallydepositing an additional layer of single crystal GaN on the surface ofthe layer of single crystal aluminum nitride; and repeating the steps ofepitaxially depositing the inter-layer of single crystal aluminumnitride and the additional layer of single crystal GaN n-times to reduceor engineer strain in a final III-N layer.
 8. The method as claimed inclaim 7 wherein the step of growing multiple layers of rare earth oxideincludes growing or depositing at least one composition layer includingmultiple rare earth oxides one of graded to bridge the multiple rareearth oxides or stepped to have an abrupt change in the rare earthoxides.
 9. The method as claimed in claim 8 wherein the step of growingor depositing the composition layer includes growing or depositing afirst rare earth oxide adjacent the silicon substrate having a crystallattice spacing substantially matching a double lattice spacing ofsilicon and a second rare earth oxide adjacent the first layer of singlecrystal GaN and having a crystal lattice spacing substantially matchinga crystal lattice spacing of the first layer of single crystal GaN. 10.A method as claimed in claim 7 wherein the step of epitaxiallydepositing the final layer of III-N material includes growing one of anLED structure or an HEMT structure on the final layer of III-N material.11. A method as claimed in claim 10 wherein the step of growing the LEDstructure on the final layer of III-N material includes growing at leastone layer including one of i-GaN, n-GaN, active layers, electronblocking layers, or p-GaN.
 12. A method as claimed in claim 10 whereinthe step of growing the HEMT structure on the final layer of III-Nmaterial includes growing at least one layer including one of, i-GaN,AlN, AlGaN, or GaN.
 13. The method as claimed in claim 10 wherein theone of an LED structure and an HEMT structure includes an LED, and theDistributed Bragg Reflector (DBR) is grown and positioned to reflectlight emitted by the LED from the substrate toward the LED.